1. Field of the Invention
The invention relates generally to low-temperature curable photosensitive compositions containing polyamic acids, and, more particularly, to low-temperature curable photosensitive compositions containing polyamic acids, which compositions are developable in aqueous alkaline solutions and are curable to low modulus polyimides suitable for use in electronic circuitry applications.
2. Background Information
A printed circuit board may be single-sided with a conductive element on one side of a base (substrate), or double-sided with a conductive element on both sides of a substrate. The conductive element, which may consist of a metal layer of copper foil for example, can be applied to the substrate (i.e., a dielectric film layer) with an adhesive. In another example, the metal can be plated directly to the substrate via electrolytic deposition. Circuit lines, typically protected by a punched coverlay or a photo-processed solder mask, are formed by removal of selected portions of the conductive element on the circuit board using photoresist radiation exposure, development and chemical etching process steps. The punched coverlay, or the photo-processed solder mask, may also function as an insulating, dielectric coating layer.
Protective dielectric coatings used to encapsulate circuitry are described in U.S. Pat. Nos. 3,469,982 and 3,547,730, may be in the form of a film layer disposed between a removable protective cover sheet and a temporary support film. After removal of the cover sheet, the film layer can be laminated on a copper substrate, imagewise exposed to actinic radiation through the support film, and developed with organic solvents or aqueous solutions to form a defined resist layer. The defined resist layer may be thereafter modified by selectively etching, electroplating, or treating with solder.
As the electronics industry is continually driven toward faster, more reliable and compact devices, there is a concomitant need for flexible materials that can withstand manufacturing processes and varying environmental conditions while maintaining integrity and stability. Suitable flexible materials are needed to accommodate limited space availability, i.e., when the circuit is folded or bent, either once, for example, in a camera or video cassette recorder (VCR), or multiple times, for example, in a computer disk drive, while retaining functionality.
There remains a need for a flexible material for use in electronic circuitry applications.